I am having a problem with a 1502, I have been running off the top outlet for a while now with no problems. recently the ehiem on the top has been cutting out making a crackling noise and the circulation stops, and the drip stops. took the ehiem off it is still working. with out the circulation, pockets of co2 build up within the media. also when the drip stops it is very hard to get it back. thought it was aqualifter but that is working. I have not completly changed out the entire media in about 6 months just been topping off could this be the problem? I took it apart and played with it I got in sink by opening the other drip, pump kicked on and every thing was fine. went back to check couple hours later no drip and crackling. tried opening the other oulet again for nothing. And I am stumped. Any idea what might be going on?